1. Field of the Invention
This invention relates to a composite amplifier incorporating a power combiner for combining a plurality of microwave signals from a plurality of amplifiers.
2. Description of the Prior Art
In a publication by E. J. Wilkinson, entitled "An N-Way Hybrid Power Divider", published in IRE Transactions (TRANS) MTT-8, January, 1960, pp. 116-118, a power divider is described that provides isolation between output terminals and approximately matched terminal impedances over about a 20% bandwidth. A plurality of quarter wavelength transmission lines are shown in FIG. 3 coupled to an input with the other end of each transmission line having a resistor coupled to a common floating node. A typical embodiment is shown in FIG. 2 wherein the transmission lines are longitudinally parallel and spaced apart from a longitudinal axis with the resistors at the end of the transmission lines radially connected in common at a node on the axis.
An amplifier with a radial line divider/combiner is described in U.S. Pat. No. 4,234,854, which issued on Nov. 18, 1980 to J. M. Schellenberg and M. Cohn and assigned to the assignee herein. In U.S. Pat. No. 4,234,854 a composite amplifier is shown comprising a radial line power divider in which a radial transmission line is divided into a plurality of sectors that are symmetrically disposed on an annular substrate, a plurality of elemental amplifiers are connected to each of the radio line sectors to amplify the power in each sector and a radial line power combiner which has a radial line transmission line divided into a plurality of sectors that are symmetrically disposed on an annular substrate are connected to the elemental amplifiers such that the combiner evenly combines the output of the elemental amplifiers in phase and amplitude to provide a composite amplified output signal.
A Wilkinson combiner/divider for splitting an input signal to two output ports is mentioned on page 120 of a publication by H. J. Hindin, entitled "Gallium Arsenide Analog ICs Rival Discrete Devices for Millitary and Commercial Use". The article is found in ELECTRONICS, Feb. 24, 1982, pp. 117-121.
At the time of the invention the inventors were facing the problem of trying to build a 3-watt, 4-stage 8-12 GHz amplifier. In order to achieve the 3-watt output power goal for the 4-stage amplifier, it was necessary to employ a large gate width, approximately 6400 micrometers field effect transistor in the final amplifier stage. Directly parallelling 32 200 micrometer cells, for example in order to obtain the required 6400 micrometer total gate periphery of the field effect transistor results in extremely low field effect transistor input impedance. Matching an impedance this low (1-2 ohms) to 50 ohms over a broad bandwidth is exceedingly difficult. More importantly, in a field effect transistor this large, phase differences between the cells close to the center feedpoint and those farthest away from the feedpoint cause a significant reduction in field effect transistor gain and power output. This is particularly true at X-band frequencies and above.
Another problem in a 4-stage wide band amplifier is that each amplifier introduces a 6 dB/octave/stage gain roll-off with frequency due to the field effect transistors. For multistage amplifiers in general, a gain equalization circuit for each field effect transistor immediately followed the particular field effect transistor. The first input of the first amplifier is flatly matched across the band, thereby retaining at least to some degree, a good input match advantage of a balanced amplifier. The final three input and last output stage are gain sloped. The equalization network therefore provides a flat drive or constant power drive across a range of frequencies to the next stage. In order to obtain gain equalization or gain slope a portion of the gain achieved by the amplifier stage is deliberately reduced at the lower frequencies.
It is therefore desirable to provide a power combiner for combining a plurality of microwave signals incorporating a modified Wilkinson N-way hybrid power combiner.
It is further desirable to provide a power combiner utilizing a modified N-way hybrid combiner wherein the combiner may be laid out on a substrate as opposed to a coaxial layout.
It is further desirable to provide a composite amplifier incorporating a Wilkinson N-way hybrid power combiner wherein path lengths to a floating node are compensated by adding capacitance.
It is further desirable to provide a composite amplifier incorporating a modified Wilkinson N-way power divider to provide input signals to a plurality of amplifiers and a modified Wilkinson N-way power combiner coupled to the outputs of the plurality of amplifiers to provide a combined output signal.
It is further desirable to provide a composite amplifier incorporating a modified N-way power divider to couple input signals to each of a plurality of amplifiers wherein a coupling network enables low impedance paths to interconnect the resistors at the floating node, thereby enabling layout patterns of spaced apart amplifiers over considerable distances on a substrate.